What is cosmological redshift?
The 2D analogy may help with this: imagine a wavy line representing a photon drawn on the surface of a 2D balloon. As the balloon expands, the wave’s wavelength will get longer and longer. The same thing happens to photons as the 3D universe blows up like a balloon: photons get all stretched out, and hence redder. [Note that stretched light has a longer wavelength, and since is the same, it must have a lower frequency.
